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Abstract:

Power measurement by tracking of a voltage cycle when voltage and current
measurements are taken at different locations on an AC power line, with
the devices taking the measurements interconnected by an asynchronous
data network. In general, a synchronization/timing message is sent from
the voltage sensing side to the current sensing side, with the
synchronization/timing message being transmitted at a predetermined point
in the periodic voltage cycle. The receipt of the synchronization/timing
message by the current sensing side may be used to re-synchronize an
internal model of the voltage cycle maintained by the current sensing
side to the measured voltage cycle. The predetermined point in the
voltage cycle may be the zero-crossing point, or other point, of the
voltage cycle.

Claims:

1. A method of measuring power comprising: measuring, by a first station
at a first location, an AC voltage on a power line having an AC voltage
cycle; causing a synch message to be transmitted from the first station
to a second station over an asynchronous network that interconnects the
first and second stations at a predetermined time in the AC voltage cycle
of the power line; the second station associated with a second location
physically spaced from the first location; detecting the arrival of the
synch message at the second station and establishing a cycle reference
time based thereon; the cycle reference time indicative of a phase of the
voltage cycle on the power line; measuring, by the second station,
current on the power line at the second location; calculating a power
drawn on the power line based on the current measured on the power line
and the cycle reference time.

2. The method of claim 1: wherein the second station has a model of the
voltage cycle on the power line; further comprising synchronizing the
second station's model of the voltage cycle to the voltage cycle based on
the cycle reference time; wherein calculating the power drawn on the
power line comprises calculating the power drawn on the power line based
on the current measured on the power line and the synchronized model of
the voltage cycle on the power line.

3. The method of claim 2 wherein the synchronizing the second station's
model comprises adjusting a phase lock loop of the second station.

4. The method of claim 2: further comprising, prior to the causing the
synch message to be transmitted, transmitting a plurality of voltage
cycle parameters associated with the voltage cycle on the power line from
the first station to the second station; further comprising generating
the second station's model of the time-varying voltage cycle on the power
line based on the plurality of voltage cycle parameters.

5. The method of claim 1 wherein the second station's model of the
voltage cycle to the voltage cycle comprises a table of values determined
prior to the detecting the arrival of the synch message at the second
station.

6. The method of claim 1 wherein the first station is a master station on
the asynchronous network and the second station is a slave station on the
asynchronous network.

7. The method of claim 1 further comprising: sending a timing alert
message from the first station to the second station over the
asynchronous network prior to sending the synch message; wherein the
synch message is the next message on the asynchronous network from the
first station after the timing alert message.

8. The method of claim 1 wherein the predetermined time in the AC voltage
cycle is a zero-crossing of the voltage on the power line.

9. The method of claim 1 further comprising periodically repeating the
steps of: causing a synch message to be transmitted; establishing a cycle
reference time; and calculating a power drawn on the power line based on
the current measured on the power line and the cycle reference time.

10. The method of claim 1 wherein the power line is a first power line,
and further comprising: measuring, by the second station, current on a
second power line at a third location separate from the first location;
after receipt of the synch message, calculating a power drawn on the
second power line based on the current measured on the second power line
and the cycle reference time.

11. The method of claim 1 wherein the asynchronous network operates
according to a MODBUS protocol.

12. A method of measuring power comprising: receiving, at a second
station, a synch message on an asynchronous network from a first station;
the start of the synch message synchronized to a predetermined point in a
regularly varying voltage cycle on a power line as measured at a first
location; establishing a reference time based on detecting the arrival of
the synch message at the second station; measuring a current on the power
line by the second station at a second location on the power line
physically spaced from the first location; determining, by the second
station, a voltage at the time the current measuring occurs based on the
reference time; calculating a first power drawn on the power line based
on the measured current and the determined voltage.

13. The method of claim 12: further comprising generating a local model
of the time-varying voltage cycle on the power line at the second
station; further comprising synchronizing the second station's model of
the voltage cycle on the power line with the voltage cycle of the power
line based on the reference time; wherein the calculating the first power
comprises calculating the first power drawn on the power line based on
the measured current and the second station's synchronized model of the
voltage cycle on the power line.

14. The method of claim 12: further comprising, prior the receiving the
synch message, the first station sending a timing alert message via the
asynchronous network to the second station; wherein the synch message is
the next message on the asynchronous network from the first station after
the timing alert message.

15. The method of claim 12 wherein the power line is a first power line,
and further comprising: measuring, by the second station, current on a
second power line; determining, by the second station, a voltage at the
time the current measuring on the second power line occurs based on the
reference time; after receipt of the synch message, calculating a power
draw on the second power line based on the current measured on the second
power line and the determined voltage.

16. A method of measuring power comprising: monitoring a voltage at a
first location on an AC power line by a first controller; the controller
in communication with a current monitor via an asynchronous data
communication network; the current monitor operative to measure a current
on the power line at a second location different from the first location;
sending a timing alert message from the controller to the current monitor
over the asynchronous network; after sending the alert message,
synchronizing the transmission of a synch message from the controller
with a predetermined point in the regularly varying voltage cycle of the
power line; the synch message being the next message transmitted from the
controller via the asynchronous network after the timing alert message;
establishing a reference time at the current monitor based on detecting
the arrival of the synch message at the current monitor; measuring
current on the power line by the current monitor; determining the voltage
of the power line corresponding to the time the current measuring occurs
based on the reference time; calculating a power draw on the power line
based on the measured current and the determined voltage.

17. The method of claim 16 wherein the power line is a first power line,
and further comprising calculating a power draw on a second power line
based on the reference time prior to receipt of a subsequent synch
message.

18. An apparatus for measuring power drawn on a power line, comprising: a
first station having a voltage sensor operative to measure an AC voltage
on a power line having a regularly varying voltage cycle at a first
location; a second station interconnected to the first station by an
asynchronous data network; the second station having a current sensor
operative to measure current on the power line at a second location
physically spaced from the first location; the second station having a
model of the voltage cycle on the power line; the first station adapted
to synchronize the sending of a synch message to the second station on
the asynchronous network with a predetermined point in the voltage cycle
as measured at the first location; the second station adapted to:
establish a reference time based on detecting the arrival of the synch
message at the second station; synchronize the model of the voltage cycle
based on the reference time; measure a current on the power line at the
second location; determine a voltage at the time the current measuring
occurs based on the synchronized model; calculate a first power drawn on
the power line based on the measured current and the determined voltage.

19. The apparatus of claim 18 wherein the first station is a master
station on the asynchronous network and the second station is a slave
station on the asynchronous network.

20. The apparatus of claim 18: wherein the second station comprises a
plurality of current sensors; wherein the second station is adapted to
measure the power drawn on a plurality of power lines based on
measurements from corresponding current sensors and the synchronized
model.

Description:

BACKGROUND

[0001] The invention relates to power measurements, and particularly to
power measurements based on voltage and current measurements taken at
separate locations.

[0002] For various reasons, it is often useful to know the amount of power
being drawn on a particular branch circuit. To do so, a measured RMS
current draw on the branch line is sometimes multiplied by a measured RMS
voltage value over one or more line cycles to calculate the commonly
known apparent power. However, it may be desirable to determine a more
accurate value of the real power being drawn, one that takes into account
the phase of the voltage cycle when an instantaneous current measurement
is taken. If the voltage and current are measured at the same time and at
the same place on the power circuit, accurately determining the power is
fairly straightforward.

[0003] However, measuring voltage and current at the same location
presents some challenges, and typically places additional restrictions on
the current measuring side circuitry that might not be present if the
current measurement were taken at a different location. For example, for
safety, fire prevention, and other considerations, it is desirable and
often mandated to maintain a high level of galvanic isolation between
energized conductors and the circuitry used to measure the voltage
between conductors and the current flowing through conductors. Measuring
the voltage on energized conductors in a manner that maintains a high
degree of galvanic isolation between the measuring device and the device
receiving the measurement is expensive, and the cost of making individual
voltage measurements when a plurality of branch circuits are to be
monitored can be high enough to preclude widespread use of such an
arrangement. In contrast, it is typically significantly simpler to
generate signals responsive to instantaneous current with high levels of
galvanic isolation, at least in situations where the current is periodic
with zero mean, because magnetic coupling can be used to generate the
current responsive signal. A understood current transformer is one
example of a commonly known device that can generate a current responsive
signal, with the electrically conductive components of the current
transformer isolated from the voltage on the current carrying
conductor(s) via a combination of air and materials which naturally
provide a high level of galvanic isolation.

[0004] In many applications, including most power distribution systems
located within an individual residence, it can be assumed that the
voltage is closely modeled as a sinusoid with equal magnitude and minimal
phase shift throughout the distribution system. As such, it may be
possible to make a single measurement of line voltage, while making
multiple measurements of the individual branch currents. However,
separating the current and voltage measurement locations presents
difficulties, particularly with tracking the relative phase information
between the voltage and current required to estimate real power.

[0005] While a number of approaches have been proposed for measuring or
estimating power, there remains a need for alternative approaches,
advantageously approaches that are adapted for situations where the
voltage and current on a given power circuit are to be measured at
separate locations, or voltage is to measured at one location with
current measured at multiple other locations.

SUMMARY

[0006] The present invention allows for tracking of voltage cycle when
voltage and current measurements are taken at different locations on an
AC power line, with the devices taking the measurements interconnected by
an asynchronous data network. In general, a timing message is sent from
the voltage sensing device to the current sensing device, with the timing
message being transmitted at a predetermined point in the periodic
voltage cycle. In some embodiments, the predetermined point in the
voltage cycle may be a zero-crossing point of the voltage cycle, that is,
a point in the voltage cycle when the sign of the voltage, measured
relative to a reference, reverses. The receipt of the timing message by
the current sensing side is used to establish a cycle reference time that
is indicative of a phase of the voltage cycle on the power line.
Typically, the cycle reference time is used to re-synchronize an internal
model of the voltage cycle maintained by the current sensing device. The
current sensing side measures current at the second location physically
separated from the location where the voltage is measured. The actual
power drawn is then calculated based on the current measured on the power
line and the cycle reference time.

[0007] In one illustrative embodiment, present invention provides a method
of measuring power. A first station at a first location measures an AC
voltage on a power line having an AC voltage cycle. The first station is
interconnected to a second station at a second location by an
asynchronous network. A synch message is caused to be transmitted from
the first station to a second station over the asynchronous network at a
predetermined time in the AC voltage cycle of the power line. The second
station detects the arrival of the synch message and establishes a cycle
reference time based thereon. The cycle reference time is indicative of a
phase of the voltage cycle on the power line. The second station measures
current on the power line at the second location. Actual power drawn is
calculated based on the current measured on the power line and the cycle
reference time. Typically, the second station has a model of the voltage
cycle on the power line. The second station's model of the voltage cycle
is synchronized to the voltage cycle based on the cycle reference time.
For example, a phase lock loop in the second station may be adjusted
based on the phase information provided by the cycle reference time. The
power drawn on the power line may then be calculated based on the current
measured on the power line and the synchronized model of the voltage
cycle on the power line. If desired, the synchronized model may be used
to determine power on multiple branch circuits based on corresponding
current measurements.

[0008] In another illustrative embodiment, the present invention provides
a method of measuring power. A first station measures an AC voltage on a
power line at a first location. A model of the time-varying voltage cycle
on the power line is generated at a second station associated with a
second location; the second location physically spaced from the first
location. For example, the first and second locations may be on opposite
sides of a breaker associated with the power line. A synch message is
caused to be transmitted from the first station to the second station
over an asynchronous network that interconnects the first and second
stations at a predetermined time in the AC voltage cycle of the power
line. A reference time is established based on detecting the arrival of
the synch message at the second station. The second station's model of
the voltage cycle on the power line is synchronized with the voltage
cycle of the power line based on the reference time. The second station
measures current on the power line at the second location. A power draw
on the power line is calculated based on the current measured on the
power line and the synchronized model of the voltage cycle on the power
line. The method may include sending a timing alert message from the
first station to the second station over the asynchronous network; with
the synch message limited to being the next message on the asynchronous
network from the first station after the timing alert message. Either the
first or second stations may be the master station on the asynchronous
network. The second station's synchronized model of the voltage cycle on
the power line may be independent of the content of the synch message.
The asynchronous network may operate according to a MODBUS protocol while
practicing the invention.

[0009] In another embodiment, the present invention provides a method of
measuring power that includes receiving, at a second station, a synch
message on an asynchronous network from a first station; the start of the
synch message synchronized to a predetermined point in a regularly
varying voltage cycle on a power line as measured at a first location. A
reference time is established based on detecting the arrival of the synch
message at the second station. The second station measures current on the
power line at a second location physically spaced from the first
location. The second station determines a voltage at the time the current
measuring occurs based on the reference time. A first power drawn on the
power line is calculated based on the measured current and the determined
voltage. The second station may generate a local model of the
time-varying voltage cycle on the power line at the second station, with
the second station's model of the voltage cycle synchronized with the
voltage cycle of the power line based on the reference time. The
calculating of the first power then includes calculating the first power
drawn on the power line based on the measured current and the second
station's synchronized model of the voltage cycle on the power line.

[0010] In another embodiment, the present invention provides a method of
measuring power that includes monitoring a voltage at a first location on
an AC power line by a first controller; the controller in communication
with a current monitor via an asynchronous data communication network.
The current monitor is operative to measure a current on the power line
at a second location different from the first location. A timing alert
message is sent from the controller to the current monitor over the
asynchronous network. After sending the alert message, the controller
synchronizes the transmission of a synch message from the controller with
a predetermined point in the regularly varying voltage cycle of the power
line, with the synch message being the next message transmitted from the
controller via the asynchronous network after the timing alert message. A
reference time is established at the current monitor based on detecting
the arrival of the synch message at the current monitor. The current
monitor measures current on the power line. The voltage of the power line
corresponding to the time the current measuring occurs is determined
based on the reference time. The power drawn on the power line is
calculated based on the measured current and the determined voltage.

[0011] The power on one or more power lines, such as on the majority or
all of the branch circuits associated with a load center, may be
determined using the above methods. Further, corresponding structures are
described. And, various aspects and embodiments are disclosed, which may
be used alone or in any combination.

[0015] The present invention allows for tracking of the line voltage cycle
when voltage and current measurements are taken at different physical
locations on an AC power distribution system, with the devices taking the
voltage and current measurements interconnected by an asynchronous data
network. To do so, both the voltage and current measuring devices
maintain an internal model of the line voltage. A timing message is sent
from the voltage sensing device to the current sensing device, with the
timing message being transmitted at a predetermined point in the periodic
voltage cycle. The receipt of the timing message by the current sensing
device is used to re-synchronize the internal model of the voltage cycle
maintained by the current sensing device to the voltage cycle as measured
by the voltage sensing device. In some embodiments, the predetermined
point in the voltage cycle may be a zero-crossing point of the voltage
cycle, that is, a point in the voltage cycle when the sign of the
voltage, measured relative to a reference, reverses.

[0016] In one illustrative embodiment, the present invention provides a
method of estimating real power. A first voltage measuring device
(station) measures an AC voltage on an energized conductor delivering
power at a first location and maintains an internal, mathematical model
of the line voltage, based on the AC voltage measurements. A separate
internal mathematical model of the time-varying voltage cycle on the
power line is maintained by a second, current sensing device associated
with a second location; the second location physically spaced from the
first location. For example, the first and second locations may be on
opposite sides of a circuit breaker associated with the AC power
distribution system. A synchronizing message is transmitted over an
asynchronous network from the first (voltage) station to the second
(current) station at a predetermined point in the voltage cycle, based on
the internal voltage cycle model of the voltage measuring device. The
time at which this synchronizing message is received by the second
(current) station is compared against the expected time of receipt of the
message based on the internal model of voltage cycle maintained by second
(current) station. The difference between the expected and actual receipt
times is used to adjust the timing of the internal voltage cycle model at
the second (current) station, with the objective to maintain the two
internal voltage cycle models in synchronous relation with each other.
The second station measures current on the power line at the second
location. A power draw on the power line is calculated based on the
current measured on the power line and the synchronized model of the
voltage cycle on the power line.

[0017] Referring to FIG. 1, a power distribution system in the form of a
load center power control scenario is illustrated, with a main power
supply cable 10, a load center 20, a main control station 40, and a
branch metering station 60. The main power supply cable 10 includes a
plurality of conductors, typically including two phase conductors 12 and
a neutral conductor 14 (which may be at earth or "ground" potential via
the technique known as bonding). As is conventional, the two phase
conductors 12 have AC voltage thereon that are 180° apart in phase
with the RMS value of the voltage between each of the two phase
conductors 12 and the neutral conductor 14 nominally equal.

[0018] The load center 20 routes power from the main power supply cable 10
to a plurality of loads 30 on respective branch circuit power lines 28.
As is conventional, the load center 20 includes an array of circuit
breakers 24 electrically disposed between the phase conductors 12 and the
branch circuit 28. Typically, each phase conductor 12 is connected with a
corresponding power bus 22, and the circuit breakers 24 each interconnect
a corresponding branch circuit 28 to the power bus 22. However, some
circuit breakers 24 may interconnect the two different phase conductors
12 to supply power to a load operating on a different AC voltage, for
example a 240V AC load. The branch circuits 28 provide power to various
loads 30, such as an HVAC unit, a hot water heater, etc., as is
conventional.

[0019] Referring to FIG. 2, the main control station 40 is operatively
connected to the load center 20 for monitoring and/or controlling the
load center 20. The main control station 40 includes a processor 44, a
voltage sensor 42, an analog-to-digital converter (ADC) 46, a phase
reference generator in the form of a phase lock loop (PLL) 48, an
asynchronous data network transceiver 50, and a wireless data network
transceiver 58. The processor 44 runs the software to control the overall
function of the main control station 40. The voltage sensor 42 senses the
voltage on one of the phase conductors 12 or between the phase conductors
12, and may take any suitable form known in the art. The voltage sensor
42 may, if desired, be disposed in parallel with the breakers 24, and
protected by a dedicated breaker 25 that does not lead to branch circuits
28. The unprotected side of the dedicated breaker 25 may be the phase
conductor 12 itself, or may be the corresponding power bus 22 inside the
load center 20. The voltage sensor 42 may be disposed in the main control
station 40 or in the load center 20, as is desired. The voltage sensor 42
typically outputs an analog signal representative of the instantaneous
voltage on the corresponding phase conductor(s) 12. The ADC 46 is used to
convert the analog signal to a digital sequence form suitable for use by
the processor 44. The sensed voltage is used to drive the PLL 48, which
is used to track the voltage phase cycle. The PLL 48 may be dedicated
circuits external to the processor or algorithmic emulations inside
processor 44 utilizing the digital sequence, as is desired. The
asynchronous data network transceiver 50 allows for data communication
between the main control station 40 and the branch metering station 60
over an asynchronous data network 56 such as a MODBUS network, as
discussed further below (MODBUS is a trademark of Schneider Automation
Inc. of N. Andover, Mass.). The asynchronous data network 56 of FIGS. 1-3
has an output circuit or line 54 and an input circuit or line 52 (from
the perspective of the main control station 40) that lead to branch
metering station 60. The wireless data network transceiver 58 allows for
data communication between the main control station 40 and various remote
locations using any suitable wireless protocol, such as a ZIGBEE protocol
(ZIGBEE is a trademark of Zigbee Alliance of San Ramon, Calif.).

[0020] Referring to FIG. 3, the branch metering station 60 is operatively
connected to the branch circuits 28 for monitoring the current load on
the branch circuits 28. The branch metering station 60 includes a
processor 68, one or more current sensors 62, one or more ADC's 64, a
phase reference generator or modeler in the form of a PLL 66, an
asynchronous data network transceiver 70, and an incoming data sense line
72. The processor 68 runs the software to control the overall function of
the branch metering station 60. The current sensors 62 sense the current
on their respective branch circuits 28. These current sensors 62 may take
any suitable form known in the art, such as a current transformer (CT).
Each current sensor 62 typically outputs an analog signal representative
of the measured current. Suitable ADC's 64, preferably with suitable
anti-aliasing functions, are used to convert the analog signals to a
digital sequence form suitable for use by the processor 68. The PLL 66 is
used to model the voltage cycle on branch circuits 28, as discussed
further below. The asynchronous data network transceiver 70 allows for
data communication between the main control station 40 and the branch
metering station 60 over the asynchronous data network 56. From the
perspective of the branch metering station 60, line 54 is an input line
and line 52 is an output line for the asynchronous data network 56. The
incoming data sense line 72 leads to the processor 68 from just upstream
of the asynchronous data network transceiver 70. The incoming data sense
line 72 allows the processor 68 to sense when a message is incoming to
the branch metering station 60, without having to examine the payload of
the incoming message.

[0021] As discussed above, the main control station 40 monitors the AC
voltage cycle. The main control station 40 uses its PLL 48 to model the
time-varying AC voltage cycle on the phase conductor(s) 12. As pointed
out above, the measurement may be on either one of the phase conductors
12, or between them, given that the relationship of the voltage cycles on
the two phase conductors 12 is fixed at 180° apart in phase, and
that both can be measured relative to the same reference, e.g., neutral.
The processor 44 then uses the PLL data to identify when a predetermined
point in the voltage cycle occurs. For example, the processor 44 may use
the PLL data to identify when the zero-crossing occurs, although any
other point in the voltage cycle may alternatively be used. For
information about how to identify the zero-crossing, attention is
directed to U.S. Patent Application Publication 2007/0085518.

[0022] When the main control station 40 needs to determine the power drawn
on a branch circuit 28, the main control station 40 sends the branch
metering station 60 a timing alert message via the asynchronous data
network 56. After the branch metering station 60 acknowledges the timing
alert message, the main control station 40 sends a synch message to the
branch metering station 60 via the asynchronous data network 56. The
synch message is transmitted by the main control station 40 such that the
beginning of transmission coincides with the predetermined point in the
voltage cycle. Thus, using the zero-crossing point as an example, the
main control station 40 holds transmission of the synch message until the
zero-crossing time, as indicated by the phase model of the PLL 48. In
addition, the synch message is the immediate next message transmitted by
the main control station 40 after the timing alert message. As such, no
other messages are allowed to be transmitted by the main control station
40, after the timing alert message, until the synch message is
transmitted, unless there is no acknowledgement of the timing alert
message by the branch metering station 60. In the event of failure to
acknowledge, the process may restart, declare an error, or take other
action as appropriate.

[0023] The branch metering station 60 may be continuously monitoring the
current on the branch circuit(s) 28, or sampling them at a high rate. In
order to determine better power measurements, the voltage on the branch
circuit 28 should be known (e.g., estimated) at the same time as the
current measurement is taken. To do this, the branch metering station 60
models the voltage phase cycle of the branch circuit 28, using the
corresponding PLL 66. However, because the branch metering station 60
does not actually measure the voltage, the voltage model may drift over
time and become out of synch with the actual voltage on the branch
circuit 28. To combat this, the branch metering station 60 uses the synch
message from the main control station 40 to re-synchronize the PLL 66 to
the voltage phase cycle as measured by the main control station 40. To do
so, the branch metering station 60 detects the presence of the next
incoming message after the timing alert message--with that message
necessarily being the synch message--using the incoming data sense line
72. The presence incoming message is detected by the processor 68 via the
incoming data sense line 72. Because this message--being the synch
message--was transmitted at a known point in the voltage cycle, the
processor 68 "knows" when this point in the voltage cycle occurred, and
can adjust the PLL 66 accordingly to re-synchronize the PLL 66 with the
voltage cycle. For example, the processor 68 can compare the time of
receipt of the synch message against the presently maintained timing of
the reference point of the internal voltage model, and can then use the
difference between the two (if any) to adjust the PLL 66 accordingly in
order to re-synchronize the PLL 66 with the voltage cycle. Then, using
the timing provided by the synchronized internal voltage model, the
processor 68 can match the current measurements on one or more branch
circuits 28 with their corresponding points in the voltage cycle, as
indicated by the PLL model, and determine the power drawn on the branch
circuit(s) 28. Thus, while the branch metering station 60 does not
measure the voltage cycle itself, the branch metering station 60
maintains a model of the voltage cycle through access to the voltage
information from the main control station 40, properly synchronized via
the asynchronous data network 56.

[0024] The calculation of the power for a given branch line may be done by
analyzing an array of data point pairs of the voltage (from the model)
and the measured current over a one cycle. The values of the pairs could
be multiplied together and summed over the cycle period to determine the
real power. Such an arrangement may be useful if the current waveform
differs significantly from sinusoidal. Alternatively, if a sinusoidal
form of both voltage and current is assumed, the phase difference between
the current and voltage could be determined by comparing predetermined
points (e.g., "rising" zero crossings) of the two waveforms, to determine
their relative phase angles. The power then could be determined simply by
multiplying the RMS voltage (supplied by voltage measuring side, or an
assumed value such as 120V) times the RMS current (derived from the
current measurements) times the cosine of the phase angle (derived from
the synchronization data).

[0025] Note that the synchronization of the PLL 66 is not dependent on the
content of the synch message, merely detection of its incoming presence
at the branch metering station 60. As such, the synchronization is not
dependent on the asynchronous data network transceiver 70 processing the
synch message. Thus, the processing delay of the asynchronous data
network transceiver 70 does not negatively affect the accuracy of the
synchronization. Furthermore, the effects of any zero-mean "jitter" in
the timing of the synch message can be minimized by appropriate PLL
design.

[0026] As discussed above, the main control station 40 monitors the
voltage of the incoming power on the phase conductor(s) 12. Because the
phase of the voltage cycle on the branch circuits 28 may be considered to
be the same as the phase of the voltage cycle on the corresponding phase
conductor 12, measurement of the voltage of the phase conductor 12 is
considered to be measurement of the voltage of the corresponding branch
circuit(s) 28. The effect of any propagation delay of the voltage cycle
between the two different locations where the voltage is measured and the
current is measured can be assumed to be negligible, even more so if one
considers the almost non-existent propagation delay difference between
the power circuits and the transmissions on the asynchronous data network
56.

[0027] The use of physically separated and galvanically isolated
measurements of voltage and current allows the current measurement
circuits to be designed less stringently, allowing for tighter spacing
between components and circuit board runs, lower cost components, and
more efficient packaging. Also, in embodiments where multiple current
measurements are referenced to a single voltage measurement location, the
duplication of voltage measurement circuitry may also avoided.

[0028] In some embodiments, the branch metering station 60 may use
assumptions about the frequency of the AC power (e.g., sixty hertz), its
RMS voltage (e.g., 120V), and the voltage profile (e.g., sinusoidal) in
order to calculate the power. In other embodiments, the branch metering
station 60 may receive information about the voltage cycle from the main
control station 40. For example, the main control station 40 may provide
the branch metering station 60 with the frequency, RMS voltage, and/or an
array of parameters describing the voltage cycle profile, so that the
branch metering station 60 may more accurately model the voltage cycle.
This information may be sent to the branch metering station 60 in
dedicated messages, on any suitable schedule, as it typically does not
vary quickly over time. Alternatively, the information may be sent as
part of the timing alert message, so as to be as fresh as possible.

[0029] In some embodiments, the branch metering station 60 performs the
calculations to determine the power drawn on a given branch circuit 28,
based on the measured current and the voltage information. In other
embodiments, the main control station 40 may perform the calculations,
with the branch metering station 60 providing the appropriate current and
phase information to the main control station 40 via the asynchronous
data network 56. For example, the branch metering station 60 may supply
an array of data points, with each row having data pairs of current and
phase angle corresponding to a plurality of measurements for a given
branch line 28 for a single current cycle on that branch line 28, and
different row corresponding to different branch lines 28. The array of
data may be transmitted from the branch metering station 60 to the main
control station 40 as conventional data over the asynchronous data
network 56.

[0030] The discussion above has been in the context of the main control
station 40 being the master of the asynchronous data network 56 and
measuring the voltage, and the branch metering station 60 being a slave
in the asynchronous data network 56 and measuring the current. However,
in some embodiments, the current side may be the master and the voltage
side may be the slave. If so, then the synchronization process may
proceed with the master (current side) sending a request to the slave
(voltage side). The request tells the slave that the response should
carry synchronization. The slave then waits for the predetermined point
in the voltage cycle and starts transmitting the acknowledgement message
in synch with the predetermined point in the voltage cycle. The master
detects the presence of the acknowledgment response via its incoming data
sense line 72, as discussed above.

[0031] In some embodiments, the branch metering station 60 may track the
current on one or more branch circuits 28 by sampling at a high rate, and
use the generally periodic changes in current as an oscillator rather
than a dedicated PLL 66. Further, in some embodiments, the main control
station 40 may, in addition to providing the voltage reference, also
measure the current on one circuit and calculate the power for that
circuit, but still share the voltage information with the branch metering
station 60 for use with the current measured for other circuits.

[0032] In the discussion above, it has been assumed that there is one
branch metering station 60. However, in some embodiments, there may be
multiple branch metering stations 60 on the asynchronous data network 56,
all modeling the voltage cycle based on the voltage/synchronization data
from a single main control station 40. Such an arrangement is believed to
work best if the main control station 40 is the "master" station on the
asynchronous data network 56.

[0033] The discussion above has used a MODBUS network as an example of an
asynchronous data network 56. It should be understood that the
asynchronous network 56 could alternatively operate according to any
suitable asynchronous network protocol. Further, the illustrative
description above has assumed that the incoming data sense line 72 taps
line 54. Such an arrangement works well in so-called four-wire MODBUS
arrangement, where each line 52, 54 comprises a pair of wires. However, a
similar arrangement for a so-called two-wire MODBUS asynchronous network
56 may have incoming data sense line 72 tapping line 52, as line 52 may
be both the input and the output line for the branch metering station 60.

[0034] The discussion above has generally been in the context of the
branch metering station 60 modeling the voltage cycle via a phase
reference generator such as PLL 66. Such an approach is believed to be
advantageous. However, other approaches to modeling the voltage cycle may
alternatively and/or additionally be used. For example, the branch
metering station 60 may model the voltage cycle using a table rather than
PLL 66. The table may contain values of voltage that are determined prior
to receipt of the synch message, based on assumptions about the voltage
cycle. For example, assuming that the voltage is 120 volts RMS at 60 Hz,
and that one wanted to take 128 samples within a cycle, a fixed table of
values representing the voltage at 128 points in the cycle (e.g., 130.2
microseconds apart for a 60 Hz cycle) could be precalculated. Assuming
that the predetermined point in the voltage cycle is the zero-crossing
point, the first value would be zero, with the next value being the
voltage 130.2 microseconds later (e.g., 5.89 volts), and so forth. The
branch metering station 60 could then take 127 current measurements at
130.2 microsecond intervals, starting 130.2 microseconds after detecting
receipt of the synch message on incoming data sense line 72, multiply
each successive current measurement by the corresponding voltage entry in
the table, sum the results, and divide by the number of intervals (e.g.,
128) to calculate the power. Note that this scenario takes advantage of
the fact that voltage is zero at the zero-crossing point (coinciding with
the receipt of the synch message); thus, the first current measurement
may be skipped because it will be multiplied by zero. Thus, the use of a
table of values corresponding to the voltage of the voltage cycle, with
the values determined prior to the receipt of the synch message, should
be considered as modeling the voltage cycle. Likewise, using the values
of the table in a fashion that aligns the table-based model with the
voltage cycle based on the reference time established by the receipt of
the synch message should be considered as synchronizing the model based
on the reference time. It should be noted that the table-based approach
may be less flexible in implementation than a PLL-based approach.

[0035] In another table-based approach, the branch metering station 60 may
use a PLL 66 to adjust the current sampling interval to match the line
frequency. For example, the processor 68 may compute the actual present
line frequency based on the PLL 66. The processor 68 may then adjust the
timing between current samples such that voltage table values more
accurately reflect the line voltage. For instance, if the frequency is
60.5 Hz rather than 60.0 Hz, the processor 68 could adjust the sampling
interval to be 129.1 microseconds. Likewise, if the frequency were 59.5
Hz, the processor 68 could adjust the timing interval to be 131.3
microseconds. The predetermined table of voltage values could then be
used as described above, with the values more properly representing the
voltage values of the voltage sinusoid at that frequency.

[0036] In some embodiments, every message transmission from the main
control station 40 may be synchronized to the predetermined point in the
voltage cycle. For example, the main control station 40 may be programmed
to hold transmission on any message on the asynchronous network 56 so
that transmission of the message coincides with the predetermined point
in the voltage cycle. In such situation, each message sent by the main
control station 40 is a synch message.

[0037] The various aspects of the various embodiments may be found
individually in various embodiments, or in any combination. Further, any
patents and patent publications mentioned above are each incorporated
herein by reference in their entirety.

[0038] The present invention may be carried out in other specific ways
than those herein set forth without departing from the scope and
essential characteristics of the invention. The present embodiments are,
therefore, to be considered in all respects as illustrative and not
restrictive, and all changes coming within the meaning and equivalency
range of the appended claims are intended to be embraced therein.